Heme and leghemoglobin (Lb) are formed in large amounts only when the free-living bacterium (R. japonicum) establishes a symbiotic relationship with its host Glycine max. During this mutual differentiation ("nodulation"), the pathways for both heme and Lb synthesis are co-ordinately activated and then inactivated. This proposal is concerned with heme and hemoglobin formation and their regulation during nodulation. We wish to ask the following questions: 1. What limits heme and Lb formation during nodule development? Is it delta-aminolevulinic acid (delta-ALA)? 2. How, and by which symbiotic partner, is delta-ALA made? 3. In the nodule, which partner's genome codes for the enzymes of the heme pathway. The answers to these questions will be of relevance in other hemoprotein-forming processes such as hemoglobin formation in erythrocytes and cytochrome c formation in eukaryotic cells. Lb formation also can be model for the specific macromolecular interaction occurring between symbiont (or parasite) and host.